BF: do not bother workarounding enum for schemaKey if there is None

Somehow I do not run into this behavior with pydantic 1.8.2 when using custom
metaclass.  Might be a sign of trouble but all tests pass, so I decided just
to condition on having "enum".   But then saw that resultant schema lacks "const"
so reworked to add "const" even if not enum.  May be that is not desired?

dandischema/models.py

@@ -170,10 +170,13 @@ def schema_extra(schema: Dict[str, Any], model) -> None:
                 # In pydantic 1.8+ all Literals are mapped on to enum
                 # This presently breaks the schema editor UI. Revert
                 # to const when generating the schema.
+                # Note: this no longer happens with custom metaclass
                 if prop == "schemaKey":
-                    if len(value["enum"]) == 1:
+                    if "enum" in value and len(value["enum"]) == 1:
                         value["const"] = value["enum"][0]
                         del value["enum"]
+                    else:
+                        value["const"] = value["default"]
 
 
 class PropertyValue(DandiBaseModel):